1. Field of the Invention
The present invention relates to a friction material to be used as a clutch facing or the like in a power transmission system of a motor vehicle, industrial machine, railway vehicle, etc., and the method of producing such a friction material. The friction material of the present invention is especially useful as a wet friction material to be used in oil.
2. Description of Related Art
Examples of a friction material for use in a facing of a wet clutch include a paper-like friction material, as disclosed in Japanese Patent publication Sho 58-47345, etc., which is composed of organic fiber as a paper like substrate and various friction modifiers, and is further impregnated with a binder of a thermosetting resin such as phenol resin for hardening purposes.
Such a friction material is, however, low in heat resistance and small in coefficient of friction, because the paper-like substrate and binder are both organic substances. To overcome this problem, various measures have been taken.
Examples of such various measures include increasing the number of friction discs and enlarging the area thereof in a clutch facing, for example. These measures, however, make the wet clutch construction complex and large, resulting in energy loss enlarging and costs increasing, too.
A copper-based sintered friction material, for example, has been tried to be applied to a wet clutch, too. Such a sintered friction material exhibits satisfactory heat resisance and pressure resistance even with a small number of friction discs and small area thereof to overcome the above-described problems. However, the coefficients of friction of many kinds of sintered metal-based friction material are frequently less than that of organic friction material. Consequently, the friction characteristics thereof is not good enough to be applied to a wet clutch of a recent motor vehicle with an increased performance.
It is an object of the present invention to ensure a large coefficient of friction when an organic paper-like substrate is used.
With a first aspect of the friction material of the present invention, a paper-like substrate composed of a fibrous material, and an inorganic binder with which the substrate is impregnated.
With a second aspect of the friction material of the present invention, the inorganic binder in the friction material of the first aspect contains two or more kinds of metal elements, inclusive of at least quadrivalent metal elements.
With a third aspect of the friction material of the present invention, the two or more kinds of metal elements in the friction material of the second aspect have different valences from each other.
With a fourth aspect of the present invention, the method of producing a friction material includes a first step of hydrolyzing at least one of metal alkoxide and organic group-replaced metal alkoxide to prepare a sol solution, a second step of impregnating a paper-like substrate of a fibrous material with the sol solution to prepare an impregnated substrate, and a third step of drying and firing the impregnated substrate. The first, second and third steps are performed in order.
It is preferable that silicone resin which is composed of a siloxane framework containing organic groups is further mixed with the sol solution.
The preferred weight ratio between metal alkoxide and organic group-replaced metal alkoxide ranges from 3:7 to 0:10.
It is preferable to perform the third step in an atmosphere containing ammonia. Furthermore, it is preferable to perform the third step under the supercritical conditions.
In addition, it is preferable that the paper-like substrate used in the second step is subjected to a hydroxyl group-introducing treatment previously.
With the friction material of the present invention, the fibrous material composing the paper-like substrate is bonded with the inorganic binder, resulting in the inorganic binder appearing in a surface of the substrate. Consequently, when friction is generated, the inorganic binder exhibits friction characteristics to increase the coefficient of bounding friction, resulting in a large coefficient of friction being obtained, as compared to the cases organic binder such as phenol resin is used.
The fibrous material adapted to compose the paper-like substrate is at least one of inorganic fiber such as glass fiber, rock wool, pottasium titanate fiber, ceramic fiber, silica fiber, silica-alumina fiber, kaolin fiber, bauxite fiber, boron fiber, magnesia fiber or metal fiber, and organic fiber such as linter pulp, wood pulp, synthetic pulp, polyester-based fiber, polyamide-based fiber, polyimide-based fiber, polyvinyl alcohol denatured fiber, polyvinylchloride fiber, polypropylene fiber, polybenzoimidazol fiber, acryl fiber, carbon fiber, phenol fiber, nylon fiber or cellulose fiber.
Examples of the inorganic binder include metal oxide such as silica, alumina, titania, zirconia or silica-alumina, and organic functional group-composite metal oxide containing groups such as methyl group or phenyl group. It is preferable that the binder contains silicone resin, too. Soft silicone resin enables both improvement of the flexibility and increase of the contacting area, resulting in further increase of the coefficient of friction. The preferred amount of silicone resin ranges from 5 to 70 parts by weight per 100 parts by weight of the total amount of the inorganic binder and silicone resin. If the amount of silicone resin is greater than the above range, the coefficient of friction decreases while the amount of silicone resin is less than theabove range, the effect resulting from the addition of silicone resin is difficult to be exhibited.
In addition, at least one or more various friction modifiers and fillers such as barium sulfide, calcium carbonate, magnesium carbonate, silicon carbide, boron carbide, titan carbide, silicon nitride, boron nitride, alumina, silica, zirconia, cashew dust, rubber dust, diatomaceous earth, talc, kaoline, magneisum oxide, molybdenum disulfide, nitrile rubber, acrylonitrile-butadiene rubber, styren butadiene rubber, silicon rubber or fluoro rubber, can be added by a proper amount.
The preferred composition ratio of these components is such that the amount of the inorganic binder ranges from 10 to 70 volume % per 100% of the total amount of the fibrous material, friction modifiers, etc. If the amount of the inorganic binder is less than the above range, the bonding strength of fibers in the paper-like substrate decreases to reduce the durability, whereas if the amount of the inorganic binder is greater than the above range, the friction characteristics decrease.
It is preferable that the inorganic binder as one component of the friction material of the present invention contains two or more kinds of metal elements, inclusive of at least quadrivalent metal elements. With this arrangement, greater coefficients of friction and higher abrasion resistance are ensured.
The reason for this effect has not been clarified. However, it can be considered that into the space defined mainly by one kind of metal elements, the other kind of metal elements are introduced to produce a proper twisting in molecules, and consequently, internal stress is generated to increase the strength of the binder framework.
It is preferable that the metal elements including at least quadrivalent metal elements have different valences. More specifically, it is preferable to include quadrivalent metal elements and tervalent metal elements, or include quadrivalent metal elements and bivalent metal elements, for example. With this arrangement, the coefficients of friction further increase and the abrasion resistance is further improved.
Examples of the quadrivalent metal elements include silicon (Si) and titanium (Ti). Examples of the tervalent metal elements include aluminum (Al), gallium (Ga) and iron (Fe), and examples of the bivalent metal elements include magnesium (Mg), calcium (Ca) and barium (Ba). In some cases, univalent metal elements such as potassium (K) or sodium (Na) can be used, too.
The preferred amount of the tervalent metal elements ranges from 0.1 to 5% of the entire metal elements in the binder in the number of atoms. Outside this range, the abrasion resistance decreases, whereas, inside this range, the abrasion resistance remarkably increases.
The preferred amount of the bivalent metal elements ranges from 0.2 to 10% of the entire metal elements in the binder in the number of atoms. Outside this range, the abrasion resistance decreases, whereas, inside this range, the abrasion resistance remarkably increases.
With the method in accordance with the present invention, in the first step, at least one of metal alkoxide and organic group-replaced metal alkoxide is first hydrolyzed to prepare a sol solution. Alkoxide such as silicon (Si), titanium (Ti), aluminum (Al), gallium (Ga), iron (Fe), magnesium (Mg), calcium (Ca) or barium (Ba) can be used as the metal alkoxide. The organic group-replaced metal alkoxide can be prepared by partly replacing the alkoxyl groups of the metal alkoxide with alkyl groups.
It is preferable that the alkoxide used contains metal alkoxide or organic group-replaced metal alkoxide of at least quadrivalent metal elements and further contains metal alkoxide or organic group-replaced metal alkoxide of different kinds of metal elements. It is more preferable that the alkoxide used contains metal alkoxide or organic group-replaced metal alkoxide of at least quadrivalent metal elements and further contains metal alkoxide or organic group-replaced metal alkoxide of metal elements of different valences.
The preferred weight ratio between metal alkoxide and organic group-replaced metal alkoxide ranges from 3:7 to 0:10. If the amount of metal alkoxide is greater than this range, the flexibility of the friction material decreases and the contact area also decreases, resulting in the coefficient of friction being reduced.
This first step can be performed by adding water to an alcohol solution of at least one of metal alkoxide and organic group-replaced metal alkoxide. Thus, a sol solution of hydroxides is formed. It is preferable to add acid or alkali or apply heat in the first step to improve the reactivity therein.
It is preferable that, in the first step, silicone resin which is composed of a siloxane framework containing organic groups is further added to the sol solution. With this arrangement, soft silicone resin composes one part of the binder of the friction material, resulting in the flexibility being improved and the coefficient of friction being further increased.
In the second step, the paper-like substrate of a fibrous material is impregnated with the sol solution prepared in the first step to prepare an impregnated substrate wherein sol of metal hydroxide filles the spaces between fibers. In the cases of various friction modifiers or the like being used, they may be added to the sol solution prepared inthe first step, or may be mixed with the fibrous material prior to the preparation of the paper-like substrate. Alternatively, they can be adhered to the surface of the substrate impregnated with the sol solution by sprinkling.
It is preferable that the paper-like substrate used in the second step is subjected to a hydroxyl group-introducing treatment. The hydroxyl group introduced enables a great improvement of the bonding strength of the paper-like substrate and binder, and a further improvement of the friction characteristics.
The hydroxyl group-introducing treatment is performed by treating with acid, for example. Inorganic acid or organic acid can be used as the acid for the above treatment. It is preferable to use organic acid such as acetic acid or oxalic acid. When the organic acid is used, upon firing in the following third step, remaining acid components are decomposed to disappear so that the influence on the friction characteristics can be ignored.
The acid treatment can be performed readily by immersing the paper-like substrate in an acid solution, or spraying the acid solution on the paper-like substrate such that the paper-like substrate is impregnated with the acid solution.
Other examples of the hydroxyl group-introducing method includes treating in an alkali aqueous solution such as an aqueous solution of sodium hydroxide, treating in a boiling water or treating in a supercritical vapor. With these treatments, hydroxyl groups can be introduced in the paper-like substrate so that the bonding strength with the binder is greatly improved, and consequently, the friction characteristics are further improved.
It can be considered that with the hydroxyl group-introducing treatment, the affinity between the metal hydroxide formed in the second step and the paper-like substrate increases, resulting in the hydroxyl groups of the metal hydroxide and the hydroxyl groups of the paper-like substrate are respectively oriented to get close to each other.
In the third step, the impregnated substrate is dried and fired, resulting in the hydroxide sol changing to oxide gel to bond fibers of the substrate firmly. In the case a large amount of organic group-replaced metal akloxide being used in the first step, the organic groups therein are oriented in the second step in such a way as to get close to the organic fibers of the substrate, and are bonded to the organic fibers more firmly in the third step, resulting in the strength being further improved. In addition, the organic groups enable improvement of the flexibility to increase the coefficient of friction.
If the paper-like substrate is subjected to the hydroxyl group-introducing treatment prior to the second step, the metal hydroxide and fibers are respectively oriented in the second step in such a way as to get close to each other, and the oxide gel and fibers are bonded firmly in the third step, resulting in the strength being further improved.
It is preferable to perform firing in the third step at 150 to 300xc2x0 C. for 0.5 to 1.0 hours. If the firing temperature is lower than this temperature range, or the firing time is shorter than this time range, the formation of oxide gel becomes difficult, resulting in sufficiently great strength being not obtained. If the firing temperature is higher than this temperature range, or the firing time is longer than this time range, organic substances are decomposed, resulting in the friction characteristics being lowered.
It is also preferable to perform the third step in the atmosphere containing ammonia. It can be considered that, with this arrangement, metal elements in the sol are partly nitrized. As a result, the coefficient of friction is further increased. The effect due to ammonia is obtained even if the content is a little. But, the maximum effect is obtained when about 10 volume % of ammonia is contained in the atmosphere in the third step.
It is desirable to perform the third step under the supercritical conditions. The supercritical conditions mean the conditions for bringing the organic substances in the substrate and sol to the state just before being vaporized, where molecular motions are very active. By applying pressure, the temperature at which the organic substances are brought into the supercritical state is elevated, and the molecular motions are further activated. Accordingly, the supercritical conditions are high-temperature and high-pressure conditions. With the high-temperature and high-pressure supercritical conditions, the decomposition of the organic substances can be prevented while the reactivity thereof can be improved, resulting in unreacted components being restrained from remaining. Consequently, the friction material having a large coefficient of friction can be produced stably.
With the friction material in accordance with the present invention, a large coefficient of friction can be ensured with the use of an organic paper-like substrate, and consequently, there is no need of piling many pieces of substrates, which has been needed with the conventional friction material. This results in disadvantages such as increase of energy loss causesd by a complicated and large-sized wet clutch construction being avoided, and the production costs becoming inexpensive.
And, with the addition of two or more kinds of metal elements, inclusive of at least quadrivalent metal elments, the abrasion resistance is further improved.
With the method of producing the friction material in accordance with the present invention, the friction material having a large coefficient of friction can be produced readily and stably with the use of the organic paper-like substrate.
Other objects, features, and characteristics of the present invention will become apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification.